In a traditional radio network comprising a base station and passive antenna system, for example, at least one connectorised 50Ω port is generally located on the passive antenna system and at least one connectorised 50Ω port is generally located on the basestation. These connectorised 50Ω ports allow signals to be passed there between via at least one 50Ω connectorised cable. The connectors on the base station are typically 7/16 DIN radio frequency (RF) connectors or N-type RF connectors.
Wireless communication systems are prone to intermodulation distortion (IMD), affecting performance of desired communications, which is often an artefact of non-linear behaviour of signal processing elements within transmitters/receivers of passive antenna systems. Intermodulation can be generated by both active components, for example solid state electronics, and passive components, for example antennae, filters and connectors.
Passive intermodulation (PIM), is a known issue in passive antenna systems and is caused by signal, or signals, undergoing an undesired non-linear mixing to generate an interference frequency component as an artefact.
Due to the non-linear nature of some signal processing elements within the passive antenna system, fundamental frequency components can become distorted, thereby leading to a decaying series of higher order harmonic frequency components in the frequency domain. If these generated (undesired) harmonic frequency components mix again with the fundamental frequencies the resultant artefact signal may fall within a receive band of that processed by the passive antenna system, they can effectively block real communications/communication channels, for example by making a base station receiver believe that a real carrier is present when there is not. Generally, the IMD components of concern are 3rd, 5th and 7th order, where the third order is of greatest signal strength and, therefore, often of primary concern.
Network operators have stipulated that antenna suppliers guarantee a certain level of PIM performance, which must be maintained over the lifetime of the supplied antennas. PIM performance can be affected after deployment by, for example, oxidation of connectors and/or printed circuit boards (PCBs). In the case of degraded PIM performance, network operators have forced many antenna manufacturers to replace entire networks of antenna installations at the antenna manufacturers cost. As a result, network operators need to test and maintain their network of antenna installations in order to highlight any degradation in PIM performance. Existing PIM testing techniques rely on the at least one connectorised 50Ω port for inserting test signals into the passive antenna system.
Passive antenna systems are often tested for PIM performance by inputting two fixed high power carrier frequencies into at least one connectorised 50Ω port and measuring the resultant IMD artefact presented on the same connectorised 50Ω port.
Regarding active antenna systems (AASs), similar PIM and IMD issues exist. However, traditional IMD testing techniques cannot be utilised as AASs do not provide at least one connectorised 50Ω port.
Referring to FIG. 1, a simplified block diagram of a traditional active antenna system 100 is illustrated. The example AAS 100 comprises a common public radio interface (CPRI) 102, for interfacing to a baseband processing unit of a cellular base station, such as a third generation partnership project (3GPP™) evolved NodeB (eNodeB). The cellular base station comprises base band circuits that perform demodulation decoding in the receive path and modulation encoding in the transmit path. Multiple-in/multiple-out (MIMO) data for example is transferred between the base station and the AAS 100 in LTE mode operation. The AAS 100 further comprises one or more of its own baseband processing circuits 104, which are arranged to perform functions including but not limited to for example system control, beamform manipulation and additional signal processing.
The AAS 100 comprises a plurality of parallel transceiver circuits 106 operably coupled via a switched coupler structure 108 to an antenna arrangement 110 comprising an array of cross-polarised antenna elements. At least one transmit module 112 and at least one receive module 114 within the transceiver 106 are also operably connected to the antenna arrangement 110, as shown. A further transceiver path 114 provides a dedicated common calibration transceiver path to a calibration transceiver 116.
In a transmit mode, the output from transmit module 112 is fed into the antenna arrangement 110 via a duplexer 118 and coupler structure 120. In a receive mode, each receive circuit 114 is operably coupled, via the coupler structure 120, to the antenna arrangement that is capable of receiving signals.
The inventors of the present invention have recognised and appreciated a desire to validate IMD and PIM performance in such an AAS, as well as provide a field-deployable test regime so that a performance over the lifetime of the AAS can be monitored.
Furthermore, the inventors of the present invention have recognised and appreciated a desire to provide self-test modes within such an AAS. In this manner, external test equipment may not be required if the AAS is serviced when installed at a cellular site. In addition, the inventors of the present invention have recognised and appreciated a desire to be able to remotely invoke self-test modes, or schedule them locally for a particular AAS, so that service personnel may not be required to visit installed cellular sites.
Also, if it is determined that the AAS is not performing correctly or within predefined performance limits, the inventors of the present invention have recognised and appreciated a desire to provide a system for an AAS that allows self-healing (e.g. determination that a problem exists and a solution to remotely and independently resolve that problem). This alleviates a need to replace the AAS in the field if there is degraded IMD performance.